1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine or the like. More particularly, the present invention relates to an image forming apparatus that forms a predetermined test pattern and transfers it to a transferring material during a time other than ordinary image forming process and then detects the test pattern so as to perform an image control such as a density control.
2. Related Background Art
Conventionally, in image forming apparatus using an electrophotography process, a control process called ATVC (Active Transfer Voltage Control) is performed in connection with transfer means using a contact electrification process. The ATVC is to cause a current to flow in a transferring portion during a non-image-forming period to determine an optimal transferring bias based on the values of the current and the voltage at that time.
An image forming process in a full color image forming apparatus utilizing a four color process and a multi intermediate transfer process will be described with reference to FIG. 9.
The apparatus shown in FIG. 9 has image forming means in the form of four image forming stations A, B, C and D for forming toner images of yellow (Y), magenta (M), cyan (C) and black (K) respectively. Each image forming station A, B, C or D is provided with processing units such as a photosensitive drum 1a, 1b, 1c or 1d, a charging roller 2a, 2b, 2c or 2d, an exposure apparatus 3a, 3b, 3c, or 3d, a developing apparatus 4a, 4b, 4c or 4d, a primary transfer roller 53a, 53b, 53c or 53d and a cleaning apparatus 6a, 6b, 6c or 6d. The above-mentioned primary transfer rollers 53a to 53d are connected with power sources for applying primary transfer bias 54a, 54b, 54c and 54d respectively.
Below the image forming stations, there is provided an intermediate transfer belt 51, a secondary transfer opposed roller 56, a secondary transfer roller 57, a sheet feed cassette 8, a feed roller 81, conveying path rollers 82, a fixing apparatus 7 and an intermediate transfer belt cleaner 55.
After the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging rollers 2a to 2d, electrostatic latent images are formed on their surfaces by exposure performed by the exposure apparatus 3a to 3d in accordance with image signals. Then, the electrostatic latent images on the respective photosensitive drums are developed by the developing apparatus 4a to 4d as toner images. The toner images on the photosensitive drums 1a to 1d are primarily transferred sequentially onto the intermediate transfer belt 51, which is rotating in the direction indicated by arrow R5, at a primary transfer nip portion T1 by the aid of primary transfer biases applied to the primary transfer rollers 53a to 53d by the primary transfer bias applying power sources 54a to 54d. The transferred toner images are superposed on the intermediate transfer belt 51.
The toner remaining on the photosensitive drums (i.e. transfer residual toner) that has not been transferred to the intermediate transfer belt 51 is removed by the cleaning apparatus 6a to 6d. 
The toner images of four colors having been transferred on the intermediate transfer belt 51 are secondarily transferred onto a recording material P (e.g. a paper sheet) at a secondary transfer nip portion T2 at one time with the aid of a secondary transfer bias applied between the secondary transfer opposed roller 56 and the secondary transfer roller 57. The recording material P is fed from the interior of the sheet feed cassette 8 to the secondary transfer nip portion T2 by means of the feed roller 81 and the conveying rollers 82 etc. The toner remaining on the intermediate transfer belt 51 (i.e. transfer residual toner) is removed and collected by the intermediate transfer belt cleaner 55.
The toner images on the recording material P are heated and pressurized in the fixing apparatus 7 by a fixing roller 71 having a heater 73 disposed in the interior thereof and a pressure roller 72 so as to be fixed on the surface of the recording material P. Thus a four-color process full color image is formed.
In the image forming apparatus shown in FIG. 9, the primary transfer means utilizes a contact electrification (or charging) process that uses transfer rollers 53a to 53d in the form of elastic rollers. This process is conventionally used in many image forming apparatus that use an electrophotography process, since it is low in cost and it does not generate ozone.
However, in the aforementioned type of transfer rollers 53a to 53d, it is difficult to suppress a variation in the electric resistance at the time of manufacturing and the resistance is liable to vary due to a change in environmental temperature and humidity or aged deterioration. With the transfer rollers 53a to 53d as such, in the case that a constant current control is effected with respect to the transfer bias so that a prescribed transfer current would always flow, the transfer voltage varies depending on the printing ratios of transferred toner images, so that in some cases, images are not be transferred optimally. In view of this, the following arrangement has been conventionally adopted in order to always realize a prescribed transfer current by a constant voltage control. That is an arrangement provided with control means that can effect both a constant current control and a constant voltage control on the primary bias applying power source and detecting means for detecting the voltage and current under those control, wherein the transfer bias is controlled by the constant current control during pre-rotation in the image forming process in which a toner image is not formed on the photosensitive drum 1a to 1d, and an optimal transfer voltage for the charge potential of the photosensitive drum 1a to 1d and the value of the resistance of the transfer roller 53a to 53d are determined, so that upon transferring a toner image, the constant voltage control is effected with the determined transfer voltage. This is a control process called ATVC, with which a necessary transfer current flow can be realized under a constant voltage control.
On the other hand it has also been performed conventionally to form a predetermined test pattern (as a toner image) during a period other than normal image forming period so that an image control such as a density control of an image would be performed by measuring the reflection density of the test pattern.
Generally, upon forming a toner image on a photosensitive drum, the toner is developed with development contrast as shown in FIG. 10. In the graph of FIG. 10, the abscissa axis represents the DC voltage of the charging bias applied to the charging roller 2a to 2d and the ordinate axis represents the surficial charge potential (surface potential) of the photosensitive drum 1a to 1d. Vd represents the surficial charge potential of the photosensitive drum 1 charged by the charging roller 2a to 2d (i.e. dark portion potential) and Vl represents the surficial charge potential of the area of the photosensitive drum that has been exposed by the exposure apparatus 3a to 3d (i.e. bright portion potential). Vdc is the developing bias applied to the developing apparatus 4a to 4d. The development contrast is, as shown in FIG. 10, the potential difference between the DC component Vdc of the developing bias and the bright portion potential Vl of the photosensitive drum 1a to 1d. There is such a correlation between the development contrast and the toner bearing amount that the larger the development contrast is, the larger amount of toner is developed on the surface of the photosensitive drum 1a to 1d. 
However, the bright portion potential Vl of the photosensitive drum 1a to 1d varies greatly depending on environmental temperature and humidity or the degree of endurance of the photosensitive drum 1a to 1d. Therefore, it is difficult to determine the development contrast precisely. In view of this, in the case that precise information on the development contrast in relation to the toner bearing amount is required as is the case upon forming a test pattern for density control, a toner image is formed, differently to the above described image formation process, by a process called analogue development in which precise information on the development contrast can be obtained.
In that process, as shown in FIG. 11, the surface of the photosensitive drum 1a to 1d is charged by the charging roller 2a to 2d up to a predetermined dark portion potential Vd and a developing bias with a DC component value Vdc larger than Vd is applied to the developing apparatus 4a to 4d with negative polarity. A negatively charged toner image is developed by the development contrast as the difference between the dark portion potential Vd and the developing bias Vdc at that time. Thus, precise information on the development contrast is obtained without an influence of the bright portion potential that is liable to vary due to changes of the photosensitive drum 1a to 1d caused by the environments or the endurance, so that it is possible to obtain a test pattern corresponding to the development contrast.
Upon detecting the toner bearing amount of the test pattern formed on the photosensitive drum 1a to 1d by means of a reflective density sensor or the like, it is difficult in the case of the image forming apparatus that uses a photosensitive drum of a small diameter to arrange the aforementioned reflective sensor for detecting the test pattern on the photosensitive drum. On the other hand, if the aforementioned reflective density sensor is to be arranged on the photosensitive drum, four reflective density sensors are required in the case of the image forming apparatus provided with photosensitive drums for four colors (i.e. four photosensitive drums). This leads to the problem of an increase in the cost. In view of the above, there has been conventionally performed a method in which a test pattern formed on a photosensitive drum is once transferred onto the intermediate transfer belt 51 and the transferred test pattern is detected by a reflective density sensor disposed in the vicinity of the intermediate transfer belt 51.
Japanese Patent Application Laid-Open No. 11-109689 discloses a method in which upon normal image formation, a transferring bias is controlled based on a change in the voltage applied to charging means. This method is to maintain an optimum transferring bias, even when Vd varies by changing the charging conditions due to change in temperature and humidity in the environment, by setting the transferring voltage Vtr in such a way that the transferring contrast between Vtr and Vd becomes always constant as shown in FIG. 12.
However, studies made by the inventors revealed that in the case that a toner image formed by analogue development is transferred onto an intermediate transfer belt 51, an optimal transferred image cannot be obtained even when the transferring bias Vtr is set in such a way that the transfer contrast between Vtr and Vd becomes constant in the manner described above.
This is because in the case of analogue development, toner images are formed in the area of the dark portion potential Vd shown in FIG. 11, while toner images developed in the normal image formation process are formed in the area of the bright portion potential Vl of the photosensitive drum as shown in FIG. 10.
Therefore, even when the transferring voltage is optimum for Vl, the transferring contrast is different for Vd with which analogue development is performed, and so the transferring of a test pattern is not performed optimally. Consequently, there is a problem that image control cannot be performed correctly.